In an optical transmission system, the power of a transmitted signal light may be changed in a transmitter for an optical signal or in a transmission path by attachment or detachment of an optical fiber or loss variations thereof. For example, in a wavelength division multiplexing (WDM) network, where optical signals having different wavelengths are multiplexed, a reconfigurable optical add/drop multiplexer (ROADM), an optical cross-connect (OXC), or other devices may be used.
In such a network, the number of input-output optical channels or the level of a signal light may be sharply changed by insertion or splitting of an optical signal, switching of an optical transmission path, or a failure, such as a break in an optical transmission fiber.
There is also a network in which an optical amplifying apparatus for collectively amplifying a WDM signal in which optical signals having different wavelengths are multiplexed is used as an optical repeater. In such a network, if the optical amplifying apparatus cannot promptly control the optical output power at a given level in response to a change in the level of an input light resulting from a change in the number of wavelength multiplexing, a variation in the signal light power occurs for each wavelength of a transmitted light.
In an optical receiving apparatus that includes an optical receiver, if the optical reception level departs from the dynamic range by such a temporary change, an in-service transmission signal may be affected and an error may occur. To address this, a configuration that may reduce an overshoot and/or an undershoot of an input into the optical receiver to the dynamic range is used.
For example, in an optical receiving apparatus of the multi-level differential phase modulation type, because a light in which a signal light that had reached an optical receiver was delayed is used as a reference light and the phase of an optical signal is detected using interference of the reference light and a signal light, the dynamic range of an optical receiver is restricted to a limited range of a high optical level.
To address this, there exists a configuration in which the signal light level is raised to the dynamic range of the optical receiver by the use of an optical amplifying apparatus disposed before the optical receiver.
An example of the optical amplifying apparatus is an erbium-doped fiber amplifier (EDFA), in which a fiber doped with erbium ions (Er3+) (erbium-doped fiber (EDF)) is used as an optical amplification medium. An EDFA amplifies the optical power of a signal light by the use of induced emission caused by an optical signal traveling in the EDF in which an excitation light output from the excitation light source is injected.
In an optical amplifying apparatus disposed before an optical receiver, auto level control (ALC), which is control for making the level constant, is carried out such that, to achieve a target limited dynamic range, the optical input power of the optical receiver is controlled so as to be a given power. Unfortunately, however, even with a configuration that carries out ALC, if the signal light power sharply varies, the gain of the optical amplifying apparatus may be unable to follow it, and the reception power of the optical receiver may depart from the dynamic range.
Another approach disclosed in the related art is the technique of detecting a recovery from a decrease in the optical input power or an optical surge and using a pass wavelength of a tunable optical filter, the pass wavelength being changed to outside the wavelength range of an optical signal, until the time when the power of a light output from the EDF returns to a steady state to prevent an exceeding optical power from entering the optical receiver. One example of that related techniques are also disclosed in Japanese Laid-open Patent Publication No. 8-331048.